Misdiagnosis happens often. Get a second or third opinion. Don't inform the new doctor about the last doctor. This way you'll get an honest opinion not one that follows the previous.
I doubt this is diabetes related. This person should be given a full evaluation by his doctor and referred to a specialist as needed. Pain pills don't solve a problem that needs to first be diagnosed. It's good that heart disease has been ruled out but you need to go from there to figure out what is going on.